The general construction of sealed, cylindrical galvanic cells is such that the principal components, an anode and cathode, are assembled into a can, together with the appropriate separators, electrolyte, etc. Generally, the can into which the principal cell components are assembled is a cathode can--i.e., the can, and usually the closed bottom end thereof, forms a cathode contact for the cell--and the cell is closed by a member placed in the top end of the can. Such member is generally a sealing and insulating member, by which the cell is sealed so as to preclude electrolyte leakage thereform, and so that the anode contact of the cell is insulated from the can at the opposite end thereof from its closed bottom end.
However, the sealing and insulating member for the sealed galvanic cell may serve several purposes, and ideally is designed so that it will assure sealing of the galvanic cell during and after deep discharge operations and/or during or after severe temperature cycling. Ordinary commercial galvanic cells such as primary alkaline cells and others are, indeed, tested to determine whether the sealing will remain integral following deep discharge or reverse polarity connections, or temperature cycling; but the severity of the cell tests may vary, depending upon the type of cell.
Moreover, it is often desired that a sealed galvanic cell should be provided having a rupturable vent membrane in the sealing member, such that if extreme pressures develop beyond a predetermined limit within the cell, the membrane will rupture thereby permitting a controlled release of gas--and, usually, some of the cell components--without the danger of the entire cell disintegrating or exploding in an uncontrolled manner.
Still further, it has always been desirable that the material of the sealing and insulating member which is placed at the top of the cell can should be substantially inert to the cell components that are within the can.
It has been the general rule, however, that not all of the above requirements have been met by any one sealing and insulating member. One of the severe problems that is encountered in the provision of a sealing and insulating member is that, normally, it would be expected that a material having substantial mechanical rigidity and tensile strength, and having co-efficients of thermal expansion very similar to those of steel--the material of which the galvanic cell can would normally be produced--should be used. Such materials generally comprise nylon. However, vinyl and polythylene have also been materials that have been widely considered and used.
Certain of the prior art has been directed toward the provision of sealing and insulating members which possess certain physical properties, and which may also provide sealing gaskets or members which have provision to permit pressure relief from the interior of the cell if the pressure therein exceeds a predetermined maximum. For example, British patent specification No. 1,517,137, published July 12, 1978, shows a sealing element which may be made of a synthetic resin material having rubber-elastic properties, and in one embodiment provides such a sealing element with a diaphragm. That patent, however, is particularly directed to a sealing element having a self-locking engagement when positioned in the cell can, and the pressure diaphragm is provided to permit pressure relief without losing the locking engagement of the sealing member with the can. The member is provided having a bore through whichan electrically conductive member can be inserted.
Southworth, in U.S. Pat. No. 3,219,488 issued Nov. 23, 1965, provides a sealing gasket member for alkaline galvanic cells, where a combined blow-out and gas-permeable membrane is provided in the sealing member. In that case, the member is generally made of nylon, and a specific embodiment is necessary so that there is a point of localized stressed for the blow-out membrane, over which a metal washer is placed.
Carmichael et al, in U.S. Pat. No. 3,218,197, issued Nov. 16, 1965, provide a similar membrane arrangement, in a gasket member which is made of a plastic that must have resistance to cold flow. However, that patent, and the others, makes no reference or consideration to sealing the cell during or after severe temperature cycling.
One other approach to the provision of a pressure vent for a sealed primary cell is that taught by Levy in U.S. Pat. No. 4,191,806 issued Mar. 4, 1980. In that case, Levy provides cell membranes in a variety of embodiments, which are injection moulded but which have grooves in the rupturable membrane portion so as to promote rupture at a generally predetermined pressure. Levy suggests that the placing of the grooves in the diaphragm permits the diaphragm to have sufficient thickness to be injection moulded in one step, and yet still to be rupturable at the desired pressure. Moreover, Levy finds that more than one groove is preferable to enable venting at a predetermined pressure, and is limited thereto.
Levy states that materials that he may use are any substantially rigid material such as polysulfone, polyphenylene oxide, polyamides, polypropylene, polyethylene, polystyrene, and acrylonitrile-butadiene-styrene. Any of those materials may be filled with glass; and the preferable materials are ABS, polyamides (nylon), and most particularly polyphenylene oxide and polypropylene.
However, as noted, Levy must provide at least two grooves in the rupturable diaphragm because otherwise, he says, it is not possible to mould the plastics so as to assure that the rupturing strength of the membrane will be such that blow-out at a predetermined pressure will occur.
We have discovered that, contrary to expectations that would be drawn from any of the prior art referred to above, the use of nylon, polyvinyl chloride or ABS--which are recognized as "engineering plastics"--does not necessarily lead to a sealing and insulating member whose integrity can be relied upon, especially following severe temperature cycling, deep discharge or reverse polarity connection, and similar abuses to which a cell might be subjected. Particularly, integrity of the seal to preclude electrolyte leakage from the cell is most difficult to attain.
However, we have discovered quite unexpectedly that, when polypropylene or certain other materials and co-polymers are used, and even when previously used materials as discussed above are used in sealing and insulating members having physical characteristics and properties heretofore not provided, the physical integrity of the sealed cell is much greater than before. Moreover, we have discovered that not only may sealing and insulating members be provided for sealed galvanic cells, such sealing and insulating members can be provided that are injection moulded and may, when necessary, be moulded with rupturable membranes formed therein during the moulding operation but which assure pressure relief venting of the cell at a predetermined pressure. These features are particularly evident when certain polypropylene mono-polymers and co-polymers are utilized, as discussed hereafter.
In providing a sealed galvanic cell according to the present invention--for example, alkaline primary cells, lithium manganese dioxide cells, and others--we have discovered that some provision should be made either for a relative change of elevation of at least part of the sealing and insulating member for the cell with respect to the bottom of the cell can, or flexing of a peripheral rib formed in the sealing and insulating member, or both, when the can is sealed by a crimping action whereby crimping forces are exerted against the material of the can to cold work the can material (usually steel) over the upper and outer peripheral portions of the sealing and insulating member. By such crimping action, the sealing member is held in sealing relationship to the can between the crimp and a bead which is formed near the top edge of the can.
However, when such materials as polypropylene are used, it must also be kept in mind that the thermal co-efficient of expansion of polypropylene is quite different than that of steel, unlike that of nylon which is much closer to that of steel. Thus, the sealing and insulating members of the present invention are provided when made of polypropylene (homo-polymers or co-polymers) or other materials as discussed hereafter, wherein cells which have the sealing and insulating members may be submitted to severe temperature cycling, storage at very high or very low temperatures, deep discharge, or combinations of them, without losing the integrity of the seal and thereby providing assurance against electrolyte leakage from the cell.
Still further, we have discovered that, when cells are finally assembled, including driving the current collector through the sealing and insulating member and jacketing the cell, there have heretofore been considerable problems in placing the current collector. Generally, the current collector has the appearance of a nail, and is driven through a central portion of the sealing member at the top of the cell. However, if the member is driven too fast, cracks may occur on the underside of the sealing member for the cell; and moreover, physical damage to the sealing and insulating member may occur such that there may be electrolyte leakage around the current collector, or alternatively such that a significant amount of the material of the sealing member is driven into the interior of the cell so as to affect its operation.
Thus, the present invention provides a sealing and insulating member for a sealed galvanic cell, which permits not only changes of the physical configuration of the sealing member when the member is crimp-sealed into the top of the cell can, but also which assures that the can remains sealed over a wide range of temperatures. Moreover, provision may be made for pressure relief venting of the sealed galvanic cell if the pressure within the cell reaches a predetermined maximum pressure.
These features, and others, are accomplished by the provision of a cylindrical sealed galvanic cell having, in combination, an anode and a cathode placed into a cylindrical cathode can which has a closed bottom end and an open top end, with an inwardly extending bead formed near the top of the can, into which is also placed a sealing and insulating member at the top of the cathode can. The sealing and insulating member is held in sealing relationship to the can by a crimp formed at the top edge of the can, with the top edge of the can extending over the top periphery of the sealing and insulating member. The diameter of the sealing and insulating member is substantially the same as the inside diameter of the can above the bead. The sealing and insulating member has a circular rib formed at its periphery in at least one of its top and bottom surfaces; and at least a first portion of the thickness of the peripheral material of the member is stressed by the crimp in an amount greater than the elastic limit of the material, and at least a second portion of the thickness of the peripheral material of the member is stressed by the crimp in an amount less than the elastic limit of the material. Preferably, the portion which is stressed beyond the elastic limit, is at the top or bottom of the periphery of the sealing and insulating member.
By providing such a sealed galvanic cell, having a sealing and insulating member as discussed above, the present invention provides a sealed galvanic cell which may withstand severe temperature or discharge conditions. Moreover, particularly when the sealing and insulating member is provided using certain homo-polymers or co-polymers of polypropylene, a venting membrane can be provided which assures pressure relief of the cell at a predetermined maximum pressure.
Still further, provision of a sealing and insulating member as spoken immediately above, and having a design at the central portion thereof according to another aspect of the present invention, is such that a current collector member may be driven through the sealing and insulating member with assurance that there will be no electrolyte leakage around the current collector member, and with the further assurance that there will not be any large portion or amount of the material of the sealing and insulating member driven into the interior of the cell so as to affect its operation.
Thus, there is provided by the present invention a galvanic cell and a sealing and insulating member therefore which may have a variety of precise configurations, depending upon the size and type of cell being sealed, but which accomplishes all of the desired characteristics and purposes described above.